Tdi Pro Coil Comparison On The Goldfields

[bobinyelm]

One commenter to the test on YouTube by Chris Ralph mentioned the Axiom has 2 settings for balancing ground for mineralization and he used only one, so best to wait for Steve to weigh in, or to specifically post in a Garrett Axiom Forum at this site or others?

I know when I spoke to Miner John, he was enthusiastic about the Axion, and he was a contributor to Whites for their design and testing, so should be knowledgeable. He suggested the Axiom as a worthy upgrade for my no longer state of the art TDI.

I certainly do not want to in any way denigrate the detector given I have not seen or used one. I am completely agnostic, and just trying to learn myself.

[GB_Amateur]

25 minutes ago, bobinyelm said:

...Best to wait for Steve to weigh in, or to specifically post in a Garrett Axiom Forum at this site or others?

Steve will see the attention notice and respond.  Starting a new thread at this point is more likely to lead to noise than signal.  😄  If Steve feels this topic needs its own thread he'll take it from there.

[Steve Herschbach]

5 hours ago, Cascade Steven said:

I am not an Axiom owner but am curious if Garrett's Axiom suffers from the same "blind spot" issue and if so  this poses a similar question as to how to fix it other than cover the same area with different settings?  Thanks

The PI “hole” is a characteristic of single channel ground balance systems as used on the TDI and its predecessor's made by Eric Foster, and any other single channel systems out there, like the QED. Minelab pioneered dual channel ground balance as a way to fill the hole. The SD 2000, SD 2100 and SD 2200 actually gave you the ability to independently balance each channel. This went away on later models with automatic ground balancing. Garrett came up with their own version starting with the Infinium, then the ATX, and the latest with Axiom. The best explanation I’ve seen of it online was by Garrett engineer Brent Weaver at the link below.

In addition, the Axiom has a proprietary “ground balance window” feature which is basically a hot rock notch setting. I’m sure this is what Chris was referring to, and why a SDC seems better if it’s tuning out rocks the Axiom is not by not using the control. The SDC is very aggressive and so it really knocks out problem rocks, but it’s not without a cost in missed gold.

[Steve Herschbach]

All ground balance systems tune out and miss gold nuggets. Gold responses and ground responses overlap. Tuning out any particular ground range or hot rock tunes out the gold that reads the same. Simply put ground balance really is just another type of discrimination system with similar issues with masking as you see with standard discrimination. Try to notch out aluminum, you will also notch out gold.

Dual channel systems reduce this ground balance “hole” issue significantly but do not eliminate it, as in the GPX 6000 example given. If you are tuning out ground, you can lose gold. That’s why applying the minimal amount of ground balance needed and digging a few hot rocks is not a bad thing. Running multiple machines over ground will almost always reveal a few nuggets missed by other machines, or taking a detector like a GPX 5000 and running multiple times with multiple settings tends to find a few missed nuggets. The GPZ 7000 probably gets more gold in a single pass than anything else due to ZVT technology, but rest assured it still misses gold that reads just like the ground it is tuning out. If you don’t tune out the ground, you can’t find the gold either since now the ground also sounds off.

Another example is tuning out saltwater. Gold and salt signal overlap. If you do not tune out the salt, wet salt ground signals continuously. Tune out the salt, now the gold that reads like salt goes away also. It actually is an issue that cannot be fixed using conductivity or signal constants as your basis of operation. Such blind spots are inherent in the technology.

• Steve’s Guide to Why Detecting Thin Gold Chains Is Difficult
• Steve's Guide to Detecting Tiny Gold Jewelry In Saltwater
• Steve's Guide to Testing Weak Gold Targets & Ground or Salt Settings

All this was a long winded way of saying that the PI hole is severe with single channel machines like the TDI, where adjusting the ground balance can actually eliminate good targets just like a disc control, and people have used it was such. Dual channels lessen the problem but do not eliminate it. So while an Axiom would be far better than the TDI in that regard, it’s not perfect, nor is any detector that rejects anything. In fact, engaging the Axiom “ground balance window” feature engages a ground balance notch with a huge range that can be set to knock out almost anything, just like with the TDI, and needs huge caution when used for the same reason

The bottom line is simple with metal detectors. Everything, everything is a trade off of sorts. Eliminating anything via filters has a dark side, one that is rarely mentioned. We focus on what we find, and we almost never know about what we are missing. 

[Steve Herschbach]

I think this chart was lost due to websites going offline, etc. It is by Reg Sniff, and attempts to illustrate the White's TDI tone shift on some targets, at different ground balance settings. Where the colored lines cross the “minimal audio change” line the object has fallen into the hole and is eliminated. At proper ground balance settings of around 8 on common ground the TDI exhibits a huge loss or inability to detect nuggets in the 1/4 ounce range, but due to nugget variability the actual loss is extremely hard to predict. See my photo chart below as to why that is.

Gold nugget target id numbers Source

[Cascade Steven]

Steve:

Thank you for your detailed explanation of this process.  Greatly appreciated.

[Jim in Idaho]

I have Reg's chart taped to the bottom of my SL. If I haven't used the SL for awhile, it's good to be able to refer to it.

Jim

[bobinyelm]

Thank you Steve for your excellent explanation of the condition!

I know you have a LOT of experience specifically with the TDI from literally years of ueu (as well as most machines around), and since that's what I am using because I cannot justify a more sophisticated PI machine right now, I was wondering how those of us still using that detector can minimize that effect and use it more effectively.

Most of us I suspect are running TDI  with GB at around 8 and delay of 10us. I know that not using GB can be used in soils that are not badly mineralized for greater sensitivity. In soils that are not highly mineralized (soil that allows effective use of a VLF machine, for instance), it is better to run on GB Off, or to in GB and use less or more than the usual "8" that balances out perfectly in most conditions, ie. purposely running a bit unbalanced)?

In other words, from your vast experience, are there settings other than those typically used that can "trick" or "fool" the detector to see things it would ordinarily not see (and mask) by varying adjustments that are favored to get a quite, stable threshold and 'quiet running'?" 

If running GB, would under balancing, or over balancing it by a, say, 1/2 or 1 unit result in better detection of small gold (at the cost of noisier or less stable operation).

Or is the best solution with the TDI just to operate in proper GB and "ALL" conductivity and accept that you're just going to miss certain size/shape/character pieces of gold and call it good until one can afford a more modern and more sophisticated detector? 

I guess if I could better visualize the chart you provided I wouldn't ask, as the chart you likely contains the answer, but on the other hand maybe there's a practical-compromise setting that summarizes the best overall performance (minimum blind spot) without multiple passes at different varying settings.

I am not addressing other PI detectors w/ regard to settings because I haven't used them, but if you have suggestions for other common machine settings to get the most from, I know that insight would be most welcome. Being able to have and use a GPZ7000 would be great to aspire to, as for now it's likely the best of the best and most capable "one-pass" unit on the market, albeit it at a price point relatively few can justify.

 

Thanks,

Bob

[Jim in Idaho]

The problem with the TDI SL, and I don't know if it exists with the other TDI models, is that the GB "range" is fairly wide. Much moreso than VLF units. I installed vernier controls on mine, so I could easily obtain a GB setting that just "barely" quiets my detector. I think it makes a difference.

Jim

[bobinyelm]

2 hours ago, Jim in Idaho said:

The problem with the TDI SL, and I don't know if it exists with the other TDI models, is that the GB "range" is fairly wide. Much moreso than VLF units. I installed vernier controls on mine, so I could easily obtain a GB setting that just "barely" quiets my detector. I think it makes a difference.

Jim

On my Gen 1 TDI, that GB range is VERY wide and poorly defined, meaning when balance is achieved (NO sound when pumping) a setting either side of that perfect balanced center is also quiet before you start getting the tone-up (when the coil is lowered when pumping) as you turn the knob counter clockwise, and going clockwise before you hear the tone-down as you lower the coil when pumping.

A hypothetical could be where pumping, the detector goes silent and no longer produces a higher tone as the coil approaches the ground (balanced) at knob position "7.5," but if you keep rotating the control clockwise, it stays silent (balanced) until you reach "8.5", at which point you start hearing the low tone approaching the ground with the coil. That 7.5 to 8.5 position is pretty wide. Maybe if I were in extremely mineralized soil, the balance knob setting becomes more critical/precise- I don't know because I have not operated the TDI in such conditions (Australia seems to have HEAVILY mineralized soil, for instance, much more mineralized than where I've tried the TDI here in Arizona).

If I read you right, you have found that the LOWEST GB knob setting (most counter clockwise) that produces silence when balancing makes the detector the most sensitive? And conversely as you rotate the knob clockwise, just as you approach hearing the tone-down when pumping is LESS sensitive? (In the hypothetical example I used above, that would mean using the 7.5 setting, as it is the lesser "barely balanced" setting you describe?)

Of course Gen 1 does not have a vernier GB, perhaps because the GB is not to touchy (critical) in finding the quiet GB position. Having not used a SL, I am guessing here.

Another question about GB: If in unmineralized soil, running GB OFF results in a single tone response to all metals, but they say it makes the detector more sensitive to deeper targets. The criteria for turning it OFF is that when attempting to ground balance, there is NO position at which you can null out the detector, meaning there is no balance point by varying the knob position w/ the GB switch ON.

If you CAN ground balance in GB ON (meaning you have mineralized soil), the manual says you "must" use GB ON. My question is, In MILDLY mineralized soil where you can find a ground balance (twisting the knob does something), but it's not so mineralized to where a VLF cannot be used effectively (indicating not BADLY mineralized ground), will running with GB OFF be productive at all, or more sensitive/deeper than with it ON?

I apologize if my questions sound "muddy," but trying to put into words in text that precisely mean what I am getting at is difficult. It's one of those situations where it takes a thousand words in print to say what you could describe in person in about 15 seconds.